[tryton-dev] Extending python-sql

['Udo Spallek' via tryton-dev]

Hi,

I would like to extend account_invoice.Invoice.search_total_amount[A]
which is based on a python-sql query.
It should be extended with another module.
In my extension I would like to replace the following column[B]::

    Coalesce(Sum(Round(
        (
            line.quantity * line.unit_price).cast(type_name),
              currency.digits)), 0).as_('total_amount'), 

with something like this::

    Coalesce(Sum(Round(
        (
            line.quantity * line.unit_price * (1 - line.discount /
    100.0) ).cast(type_name), currency.digits)), 0).as_('total_amount')


The general boilerplate code in the other module looks like::

    class Invoice:
        __metaclass__ = PoolMeta
        __name__ = 'account.invoice'

        @classmethod
        def search_total_amount(cls, name, clause):
            
            result = super(Invoice, cls).search_total_amount(
                name, clause)


With introspection I found the column::

        columns = list(
            result[0][2].columns[0].table.select.im_self
            .queries[0].columns)

But I have no idea, how to replace the column, because I have not the
correct line object from the depends module.
When I try something like this::

    pool = Pool()
    Line = pool.get('account.invoice.line')
    line = Line.__table__()

    columns[1] = Coalesce(Sum(Round((
        line.quantity * line.unit_price * (1 - line.discount / 100.0)
    ).cast(type_name), currency.digits)), 0).as_('total_amount')

    result[0][2].columns[0].table.select.im_self.queries[0].columns = \
        tuple(columns)

the line table is wrong, I need the table from [B].

Questions:
1. Is it a good practice trying to extend python-sql?
2. Is there a better way to introspect the python-sql then this::

        columns = list(
            result[0][2].columns[0].table.select.im_self
            .queries[0].columns)
3. How to get the line-object from the base sql?

TIA and Regards Udo Spallek

[A]http://hg.tryton.org/modules/account_invoice/file/tip/invoice.py#l741
[B]
http://hg.tryton.org/modules/account_invoice/file/tip/invoice.py#l764

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